In television transmission systems, either black and white or color, the video and audio information along with synchronizing signal components are transmitted as part of a composite signal. It is desirable to include additional information beyond the picture and sound information presently transmitted for the purpose of channel identification, subtitles for deaf persons, special notices and the like. A number of proposals have been made for adding auxiliary signals to the present composite television signals to convey this additional information.
In order to conserve the limited frequency spectrum space which is available for the transmission of signals in the portion of the spectrum allocated to television transmission, it is desirable to transmit such auxiliary information within the same frequency band already occupied by the present composite television signal alone.
Auxiliary signal systems have been devised for permitting the transmission of information in addition to the conventional video and sound signals. Most of the prior art systems result in noticeable degradation of the video information carried by the television signal, do not operate at a sufficiently high data rate to be very useful in conveying the additional information, or are unreliable.
Some prior art systems use an approach in which entire frames or fields of the television signal are replaced with frames or fields of an auxiliary image representative signal which may be detected and displayed in a separate receiver. Such systems, however, are subject to a substantial data rate limitation. In addition, they cause serious degradation of the normal television signal, because the removal of the normal signal, and its replacement with the auxiliary signal, interferes with the reproduction of the normal signal on the cathode ray tube of the receiver.
Another technique which has been employed is to add auxiliary signal information to the television signal during the blanking intervals. This is known as a time sequencing system, and, once again, it is subject to substantial data rate limitations or reliability problems because of the very short time available during the blanking intervals for transmitting the auxiliary information.
A system has been proposed for transmitting an add-on signal with the frequency band occupied by the normal television signal, where, during successive line intervals, the add-on signal is reversed in polarity or phase and is identically repeated during such successive line intervals. Thus, the add-on signal tends to visually cancel when the composite received signal is processed and displayed by a conventional television receiver. A separate add-on signal processor then is provided for alternately inverting successive lines of the received composite signal to display the add-on signal and cancel the normal television video signal from the display of the add-on processor. This system, however, requires the additional display unit of the add-on processor along with a separate signal processing system which duplicates to a large extent of the system of a conventional television receiver. This system does, however, have an advantage inasmuch as degradation of the conventional or normal television signal reproduced by television receivers is minimized.
Another prior art system which has been employed utilizes a 3.58 Mhz sub-carrier signal (the conventional color sub-carrier signal) to convey auxiliary information in a black and white television receiver or to carry the additional auxiliary information in a color television receiver. Circuitry is employed for detecting the auxiliary signal information and adding it to the normal video information in the receiver. The auxiliary information, however, is simultaneously displayed along with the conventional video information, so that substantial visual interference between the two exists on the face of the cathode ray tube of the television receiver. As a consequence, in order to have a highly visible auxiliary signal which overrides the conventional video display, it is necessary to employ high amplitude auxiliary signals. When this is done, the possibility of increasing the interference between these signals and the desired conventional television signals is substantially increased.
It is desirable to add auxiliary video signals to the normal or conventional television signal for display on the face of the television receiver cathode ray tube on deman, where the auxiliary signals are transmitted within the same bandwidth as the conventional signal, and where degradation of the conventional television signal is minimized during periods when the auxiliary signal is not being displayed.